This invention relates, in general to antennas, and in particular, to printed circuit board antennas providing polarization and pattern diversity.
Printed circuit board antennas have been used for many years in military systems and have recently found application in commercial communications, such as wireless Local Area Networks (LANs), in which small physical size is a key requirement. Printed circuit board antennas are elements used for transmission or reception of radio waves in the UHF and microwave/millimeter wave spectra for high frequency communication systems. In these systems the operating wavelengths are sufficiently short to accommodate conveniently small geometries available in planar antenna elements. In wireless LAN applications, an indoor environment is typically encountered wherein the propagation of radio waves from one point to another can be greatly affected by the surrounding structures of office areas, and changes arising from activities taking place within the office area.
Undesired scatter of radio waves from reflective surfaces can promote a condition referred to as multipath interference, which can severely degrade radio signal strength and prevent radio communication from taking place. Solutions to the problem of multipath interference typically make use of the behavior of radio waves and the manner in which antennas respond to them. One solution to improve the response of an antenna to the degradation due to multipath interference is to displace the antenna a distance of approximately one half wavelength from a point in which destructive interference most severely degrades signal intensity. The interference due to multipath can be constructive rather than destructive at such a distance, but this involves physically moving a unit which contains the displaced antenna. Moving the unit containing the antenna is not always feasible if the unit is a desktop computer which is situated in a location that best accommodates a user.
An accompanying solution, which makes use of the concept of locating the antenna in a favorable location, is known as space diversity. This technique utilizes switching between antennas placed in different locations. When propagation conditions in the operating environment change to favor operation with one antenna over another, a switching network is used to select that antenna. This technique makes use of multiple separate antennas spaced apart, but interconnected with switching electronics and cables which take up room, and add cost and complexity to the antenna system. Altering the antenna system characteristics has also been a technique employed to overcome problems caused by multipath interference. One technique which may be used is pattern diversity. Pattern diversity provides an antenna system with alterable electromagnetic field pattern characteristics in any particular plane of its three dimensional far field pattern. This can be accomplished by several means including array phasing or switching from one antenna to another. Either of these methods require more than one antenna radiating element, and generally greater area than a single antenna. The constraints represented by Personal Computer Memory Card International Association (PCMCIA) form factor dimensions render these techniques impractical.
Another technique which has been applied is referred to as polarization diversity. Polarization diversity provides an antenna system with alterable polarization characteristics. This allows the antenna system response to radio waves of different polarizations to be controlled for either maximum or minimum response. This is useful as a solution for multipath interference as well as applications in line of sight (LOS) communications which require the capability of electronically altering antenna system polarization to avoid having to orient the unit containing the antenna, such as a desktop computer, orthogonally so as to correct antenna polarization with respect to that of some access point or base station. Since polarization diversity can be achieved by forcing the field components between two modes of operation to be orthogonal, switching between two antennas which are oriented in a mutually perpendicular fashion has been used. This also makes use of two different antennas, and typically requires greater area than is occupied by a single antenna. The increased area requirement also renders this technique impractical for PCMCIA form factor constraints.
Thus, it would be desirable to provide high speed wireless LAN products with an antenna element which makes use of one or more types of antenna diversity as a means of adaptability to an operational environment where multipath interference is present, and conforms to PCMCIA form factor packaging limitations.